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 The development of the
voltage-controlled amplifier for professional audio use
has a long history, and can provide some insight into the
care and feeding of these essential devices. The
following history was compiled from the collective memory
of the THAT Corporation staff, and is believed to be a
reasonably accurate, abbreviated summary of the evolution
of VCAs since the early days of dbx.
The
voltage-controlled amplifiers found in most recording
consoles and analog effects products are based on the
"Blackmer gain cell" developed by David
Blackmer of dbx, Inc. The original dbx 202 "Black
Can" VCAs, which can still be found in operating
consoles today, have been around for over 20 years. These
first VCAs suitable for pro audio equipment were built
with discrete transistors that were individually matched
in an oven. They were pretty noisy, and generated
significant distortion at higher levels, but far exceeded
the performance of other VCAs at that time, and became
quite popular for early console automation.
There have
been a number of succeeding 202 VCAs, with each series
evolving sonically as the state of the VCA art has
advanced.
The earliest designs required trims for
both transistor symmetry and log-slope. The next
generation of dbx gain cell, developed by Gary Bergstrom,
was made with pre-matched, discrete devices, and used a
patented error correction scheme which improved its
dynamic range. Like earlier designs, it suffered from
thermal tracking problems and a limited dynamic range.
Around
this time, users felt that the distortion levels of the
VCAs were too high, and for a while VCA development
centered on Class A designs. Competitors of dbx, Inc.,
including Paul Buff of Allison Research (and later Valley
People), and David Baskind and Harvey Rubens of VCA
Associates, fielded their own approaches, all aimed at
overcoming the limitations of Blackmer's early design.
The Allison Research VCA was much like a Blackmer VCA
running Class A, and had an exponential control port.
Allison Research, in fact, paid a royalty to dbx, Inc.
for certain aspects of this design. The VCA Associates
VCA did not have an exponential control law, but had
better thermal characteristics, since it was fabricated
as an IC. It is not clear if any of the VCAs are still in
production at this time.
About the
same time, dbx, Inc. introduced its own Class A design,
the 2001, designed by Bob Adams, now of Analog Devices.
This was a discrete implementation like the Allison VCA,
and also had an eight transistor gain cell like the
Allison VCA, though the 2001 used these additional
transistors in an active mode whereas the Allison VCA had
them diode connected.
These
Class A gain cells eliminated the need for a log-slope
trim by virtue of their eight transistor architecture,
but they presented significant thermal tracking problems
in discrete form, and the eight transistor gain cell did
not reach its full potential until it was fabricated in
an IC process, where matching and tracking can be held to
tight tolerances. Concurrent with the development of the
2001, Dave Welland of dbx, Inc. was working on an IC
solution that implemented a Class AB design of the eight
transistor gain cell and used the additional transistors
to implement an improved correction scheme. These IC
VCAs, the dbx 2150 et al, were current-in/current-out
devices, and it soon became obvious that they could be
paralleled. The open frame modular 202 series from dbx
(and subsequently THAT Corporation), were built in this
manner, and exhibited superior noise performance compared
to the older varieties.
With the
lower noise floor, however, other limitations became
apparent, particularly in product designs that did not
use an adequate degree of care in dealing with the
peculiarities of those early VCAs. As a result, users
began to notice certain subtle, audible anomalies that
became identified with VCAs. Some of this
"characteristic" sound no doubt resulted from
the limited gain-bandwidth product of the op-amps inside
the VCA (which in turn resulted from the relatively high
parasitic capacitances typical of the junction isolated,
complementary bipolar process used to fabricate the core
ICs). At the same time, designers occasionally failed to
adequately condition the signal feeding the control port
of the VCA, resulting in excessive noise modulation and
distortion.
In the
meantime, the ownership of dbx, Inc. changed hands
several times, and the various divisions of dbx, Inc.
were individually sold during the dissolution of Carillon
Technology Incorporated. The professional products
division was sold to AKG which was later acquired by
Harman. OEM products were bought by THAT Corporation,
founded by dbx's V.P. of Engineering, and members of his
staff, and THAT became the source of the 202 VCAs.
To keep
VCAs up to date with current technology, THAT Corporation
began development on successors to dbx's old line of
VCAs. To overcome the problems related to limited GBW
product, THAT Corporation's newest IC VCAs, the 218X
series of VCAs, was developed by Gary Hebert in a
dielectrically isolated process, and has topological
enhancements that minimize the effect of non-linearities
in the VCA drive circuitry on high frequency distortion.
These VCAs are equal or superior to their predecessors in
every regard, and in particular, do not exhibit the
rising distortion at higher frequencies characteristic of
VCAs built on a junction isolated process.
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